The advantage they cite is protection from seasonal scarcity due to needing less vitamin C. So during resource deprivation there is a window where the mutant only suffers from conventional malnutrition, whereas the non-mutant's mechanisms for creating endogenous vitamin C are also impacted. So the non-mutant suffers from both malnutrition AND scurvy, which presumably kills them faster. Eventually, the mutant would also suffer from scurvy.

That's mostly speculative though, there would need to be some kind of starvation study done between species to figure out which one dies first. That would be unethical :(